What is BMI and Body Fat Percentage?

A basic guide on health has long since been the BMI index. It means body mass index and is meant to be a quick reference guide that allows you to see how in or out of shape you really are. However the BMI scale was first created somewhere between 1830-1850 Belgian polymath Adolphe Quetelet. Originally physicians used it to calculate how much sedative could be used on a patient.

Today it is a vague measure on whether a person is overweight or underweight. This is a guide based on your weight (in kgs) divided by your height squared (in meters). This does not take into consideration your bone density or muscle mass. A body builder may have a high BMI but it is not because they are “overweight” or carrying too much fat. Generally it can be used as a quick rule of thumb measurement for overall body mass. A normal BMI on average is 18.5-25 however many people may appear to be the wrong weight because it does not take into consideration other factors.

Use the BMI as a rough and vague way of quickly measuring what kind of shape you are in. And round does not count as a shape…

bmi graph


Anyone who has largely high or low scores should seek medical advice before embarking on a new eating plan.

Body fat percentage is a more accurate reading of your mass. It is a separation of your pure fat (adipose tissue) from the mass of your bones and organs. Levels of fat differ for age and gender. Infants or older individuals will carry more fat percentage. Females are also prone to carry higher fat amounts due to the metabolic needs of supporting pregnancy.

When it comes to actual body fat measurements, it is a far better indicator of health than the BMI. Measurement techniques may be subject to some error. The more accurate tests tend to be very pricey however, thus many people will not use them.

Having fat in your body is essential for growth and maintenance of all cells. Below is a table describing different percentages of body fat and what it means for your health. A fat percentage lower than the “essential level” can have as many negative health effects as being considered overweight or obese.

body fat percentage* Essential fat is the smallest level in the body we should have before we suffer from health problems. In the case of women, too little body fat can lead to menstrual problems and/or sterility.


fat percentage idea

Body Fat Measuring Systems

Underwater weighing

With a well engineered weighing system, body density can be determined with great accuracy by completely submerging a person in water and calculating the volume of the displaced water from the weight of the displaced water. A correction is made for the buoyancy of air in the lungs and other gases in the body spaces.

Estimation of body fat percentage from underwater weighing (or hydrostatic weighing) has long been considered to be the best method available, especially in consideration of the cost and simplicity of the equipment. Most other ways to estimate body fatness, such as by skin folds, body girths, body impedance, air displacement volume and body scanners are based on equations which predict body density. In contrast, underwater weighing gives an actual measurement of body density rather than a prediction.


Bioelectrical impedance analysis

The general principle behind BIA: two or more conductors are attached to a person’s body and a small electric current is sent through the body. The resistance between the conductors will provide a measure of body fat between a pair of electrodes, since the resistance to electricity varies between adipose, muscular and skeletal tissue. Fat-free mass (muscle) is a good conductor as it contains a large amount of water (approximately 73%) and electrolytes, while fat is anhydrous and a poor conductor of electric current. Factors that affect the accuracy and precision of this method include instrumentation, subject factors, technician skill, and the prediction equation formulated to estimate the fat-free mass.

Each (bare) foot may be placed on an electrode, with the current sent up one leg, across the abdomen and down the other leg. (For convenience, an instrument which must be stepped on will also measure weight.) Alternatively, an electrode may be held in each hand; calculation of fat percentage uses the weight, so that must be measured with scales and entered by the user. The two methods may give different percentages, without being inconsistent, as they measure fat in different parts of the body. More sophisticated instruments for domestic use are available with electrodes for both feet and for both hands.


Skin fold measurements

The skin fold estimation methods are based on a skin fold test, also known as a pinch test, whereby a pinch of skin is precisely measured by calipers at several standardized points on the body to determine the subcutaneous fat layer thickness. These measurements are converted to an estimated body fat percentage by an equation. Some formulas require as few as three measurements, others as many as seven. The accuracy of these estimates is more dependent on a person’s unique body fat distribution than on the number of sites measured. As well, it is of utmost importance to test in a precise location with a fixed pressure. Although it may not give an accurate reading of real body fat percentage, it is a reliable measure of body composition change over a period of time, provided the test is carried out by the same person with the same technique.

Skin fold-based body fat estimation is sensitive to the type of caliper used, and technique. This method also only measures one type of fat: subcutaneous adipose tissue (fat under the skin). Two individuals might have nearly identical measurements at all of the skin fold sites, yet differ greatly in their body fat levels due to differences in other body fat deposits such as visceral adipose tissue: fat in the abdominal cavity. Some models partially address this problem by including age as a variable in the statistics and the resulting formula. Older individuals are found to have a lower body density for the same skin fold measurements, which is assumed to signify a higher body fat percentage. However, older, highly athletic individuals might not fit this assumption, causing the formulas to underestimate their body density.